When fishing near or on the sea bottom, it may be very important to know the distance between the fishing tackle and the sea bottom.
For pelagic tackles, this may be very important in order to avoid bottom contact. For tackles constructed for fishing on the bottom, correct bottom contact will be crucial for fishing efficiency.
A known technique for determining the distance between the tackle and the sea bottom is to use an echo sounder which is attached to the tackle and which measures the distance to the bottom. However, very short distances which gradually lead to physical bottom contact may be difficult to detect with great reliability with an echo sounder because the echoes from the tackle and the bottom may be difficult to distinguish. In addition, echo sounders for this purpose tend to be very complicated and expensive.
It is previously known from U.S. Pat. No. 4,873,523 to use a weight at the end of a rope which is kept taut when the weight is not in contact with the bottom but which becomes slack when the weight hits the bottom, which event is recorded by the surface vessel. However, this principle of detection is not reliable when dragging tackle above the bottom because the frictional forces between the weight and the bottom may now and then cause tension in the rope.
The present invention eliminates this source of error.